I think you're overstating the barrier to entry of grocery stores in general. There are small, independent competitors that are able to provide a comparable level of service. Where the big ones win out is their ability to afford extremely large retail spaces for big-box supermarkets which cater to a car-dependant, suburban lifestyle. The flaws of our low-density and inefficient city planning haunt us in numerous ways, one of which is in giving an advantage to businesses which pass the cost of convenient access on to the customer (in the form of requiring them to drive to a sprawling commercial area with a massive parking lot which has been segregated from residential zones).

I say this as someone who never shops at the big supermarket chains, as I live less than 2 minutes' walk from a neighborhood corner store which sells 90% of the groceries I need. But the sort of walkable, mixed-use neighborhood I live in is basically illegal to build nowadays due to market distortions caused by zoning laws. Zoning laws which are the result of lobbying by suburban housing developers, as well as the fossil fuel and automotive industry.